// 测试链接：https://www.luogu.com.cn/problem/P1163
// 提交以下的所有代码，可以直接通过

#include <bits/stdc++.h>

using namespace std;

const int MAXN = 1010;
int w0, w, m;

bool check(double x)
{
    double s = w0; // s为未还的钱
    for(int i = 1; i <= m; ++i)
    {
        s = s * (1 + x) - w;
    }
    return s >= 0; // x越大，s越大
}

double compute()
{
    double l = 0, r = 10;
    double ans = 0;
    while(r - l > 1e-5)
    {
        double mid = (l + r) / 2;
        if(check(mid))
            r = mid;
        else
            ans = mid, l = mid;
    }
    return ans;
}

int main()
{
    scanf("%d%d%d", &w0, &w, &m);
    printf("%.1f\n", compute() * 100);

    return 0;
}